Answering questions after addressing about 1,000 military and industry leaders with a common interest in missile defense, the secretary said proliferation is an issue no country can deal with on its own.

"By its very nature, it requires the cooperation of lots of nations," he said, "and regrettably, the United Nations has antiproliferation activities, but they're not sufficiently effective."

The reality, Rumsfeld said, is that countries trade off their comparative advantage. "They may know more about missile technology than another country," he explained, "but another country may know more about nuclear technology or biological technology, and they trade off. A third country may know more about how to deny and deceive modern countries from knowing what's taking place in their countries, and they'll trade off that comparative advantage.

"So here we are in an era where the weapons are increasingly more powerful and lethal," he continued, "(and) the dangers from them to large numbers of innocent human beings are increasingly greater."

The ease of spreading the various technologies also continues to grow, he noted. "We've got to do a much better job of working with other countries," the secretary said.

Dealing with any single country is difficult enough, he said. "All of these countries have relationships that go back many, many years," he explained. Italy, for example, has been doing business with Libya for 2,000 years, the secretary said, and Russia's relationship with Iran is different from the one the United States has with that country.

"So there always are reasons for a country to want to continue to deal with some country that the broader community would prefer not (to deal with) in terms of these technologies," he said.

The biggest single problem in working with other countries, Rumsfeld said, is "we're working off a different sheet of music" when it comes to threat assessment.

Countries that generally agree on what the threat is and share intelligence then behave in roughly the same way, he explained. "To the extent that you have a different threat assessment, or your population has a different threat assessment and you're a politician," he continued, "and you feel compelled to be with your population, it takes a long, hard task to teach people, to lead people, to educate people."

Turning around the population's view can be done, but it takes time and energy, Rumsfeld said. "And you can only do it with so many issues at once," he added.

The United States and its friends and allies need to do a better job of coming to a more common threat assessment, the secretary said. "I mean, facts are facts this isn't something complicated," he added.

Despite efforts toward cooperation, Rumsfeld noted, there always will be countries that try to work out a separate peace. "There have been appeasers throughout history," he said, "so we can't be surprised when that happens from time to time. But they tend not to be the big countries; they tend not to be the countries that have the capability to really do something about proliferation."

The secretary cited the Proliferation Security Initiative as a "good start" toward fighting the spread of weapons of mass destruction. That effort, begun by President Bush in May 2003, involves partnerships of countries working together to develop a broad range of legal, diplomatic, economic, military, and other tools to interdict shipments of weapons of mass destruction.